Examples of a transmission line that transmits a high-frequency signal include transmission lines for planar circuits, such as a microstrip line and a coplanar line. There is a high-frequency circuit board in which such transmission lines are formed on a dielectric substrate. Furthermore, there is a high-frequency circuit board in which a waveguide is formed in the board in addition to the transmission lines for planar circuits (see Japanese Unexamined Patent Publication JP-A 2002-289737, for example). When electronic components, such as an MMIC (Microwave Monolithic Integrated Circuit) and a passive electronic component, are mounted on this high-frequency circuit board, a high-frequency circuit module is implemented. A high-frequency circuit board on which a waveguide is formed can transmit a high-frequency signal in a millimeter wavelength region (e.g., 77 GHz) at a low loss, and thus, this circuit board can be used in a high-frequency circuit module of, for example, a millimeter wave radar for preventing a vehicle from colliding.
As an electronic component has more functions, the amount of heat generated in the electronic component tends to increase. Thus, there is a demand for a high-frequency circuit board that has a reduced size, and that can allow an electronic component to stably operate, by effectively radiating heat generated in the electronic component.
In a conventional technique, an electronic component is provided with a heat sink in order to radiate heat generated in the electronic component as quickly as possible (see Japanese Unexamined Patent Publication JP-A 2003-179181, for example). Furthermore, there is a high-frequency circuit board in which an electronic component is mounted on one surface of the board, a heat-radiating plate is disposed on the other surface of the board, and a through-conductor that connects the electronic component and the heat-radiating plate is formed in the board (see Japanese Unexamined Patent Publications JP-A 8-330696 (1996) and JP-A 2001-284803, for example). In this sort of high-frequency circuit board, heat generated in the electronic component moves through the through-conductor to the heat-radiating plate, and is radiated from the heat-radiating plate.
In the case where a heat sink is attached to an electronic component, for example, there are problems in that the number of steps increases because a step of attaching a heat sink is necessary after the electronic component is mounted, and in that the degree of freedom in design is reduced because a space for attaching a heat sink has to be secured.
Furthermore, in the case where a through-conductor for radiating heat is disposed in the board, both the waveguide and the through-conductor are formed in the board. Accordingly, these constituent elements cross each other in a complex manner in the board, and thus, there are problems in that the degree of freedom in design is reduced, and in that the size of the board increases.